Most pristine known asteroid is denser than granite

ASTEROIDS are often battered and bruised, mere piles of rubble held together by gravity. Asteroid 21 Lutetia, however, seems to have sailed serenely above the fray – a pristine block of rock that has remained nearly untouched since the solar system’s birth.

The European Space Agency’s Rosetta probe zipped past Lutetia in July 2010 on its way to a comet. Rosetta’s measurements suggest the 121-kilometre-long asteroid is denser than granite. Lutetia’s exterior, which occasionally gets bruised in minor impacts, is probably rocky, while its interior is made of heavy metals.

That suggests the asteroid must have melted in the past, allowing the heavy elements to sink inwards, say Holger Sierks of the Max-Planck Institute for Solar System Research in Katlenburg-Lindau, Germany, and his colleagues (Science, DOI&colon; 10.1126/science.1207325).

Aluminium-26, which has a half-life of 700,000 years, could have provided the central heating, assuming the asteroid formed within the solar system’s first million years, before the aluminium decayed. “It’s really a remnant from the early days,” says Sierks.

“We think planets were built of things like Lutetia,” says Ben Weiss of the Massachusetts Institute of Technology.

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